An aircraft is moved by several turbojets each housed in a nacelle also containing a set of ancillary actuation devices associated with its operation and performing various functions when the turbojet is operating or stopped. These ancillary actuation devices comprise in particular a mechanical system for actuating thrust reversers.
A nacelle usually has a tubular structure comprising an air intake at the front of the turbojet, a mid-section designed to surround a fan of the turbojet, an aft section that can contain thrust-reversing means and designed to surround the combustion chamber of the turbojet, and is usually terminated by an exhaust nozzle the outlet of which is situated downstream of the turbojet.
Modern nacelles are often designed to contain a bypass turbojet capable of generating via the airfoils of the rotating fan a flow of hot air (also called the main flow) originating from the combustion chamber of the turbojet.
A nacelle usually has an external structure which defines, with a concentric internal structure, an annular flow channel (also called an annular ducting channel), also called a stream, designed to channel a flow of cold air, called bypass air, which travels on the outside of the turbojet. The main and bypass flows are exhausted from the turbojet via the rear of the nacelle.
Each propulsion assembly of the aircraft is therefore formed by a nacelle and a turbojet, and is suspended from a fixed structure of the aircraft, for example beneath a wing or on the fuselage, via a pylon or mast attached to the turbojet or to the nacelle.
The aft section of the nacelle is usually formed by a first half-shell and a second half-shell of substantially semicylindrical shape, on either side of a longitudinal vertical plane of symmetry of the nacelle, and mounted movably so as to be able to be deployed between a working position and a maintenance position for the purpose of providing access to the turbojet. The two half-shells are usually mounted pivotingly about a longitudinal shaft forming a hinge in the top portion (at 12 o'clock) of the nacelle. The half-shells are held in the closed position by means of locking devices placed along a junction line situated in the bottom portion (at 6 o'clock).
The mid-section and aft section are, in conventional manner, connected to one another via a frame that is fixed relative to the turbojet, the first and second half-shells usually being fitted with positioning means interacting in the working position with matching positioning means arranged on the frame.
A nacelle of this type has the drawbacks explained below.
During maintenance operations, the two half-shells are separated from one another so that the positioning means of the half-shells no longer interact with the matching positioning means of the fixed frame.
In this case, the half-shells can be moved in translation along their axis so that, when the nacelle is closed, that is to say when the half-shells are brought together, the positioning means of the half-shells are no longer placed facing the matching positioning means of the frame.
This can cause damage to the positioning means which are furthermore parts called sensitive parts, subjected to great stresses in operation, that is to say during the flight of the aircraft fitted with the nacelle. If these positioning means are damaged too much, the half-shells may be caused to move during flight under the effect of the stresses that they sustain and, in this case, lead at the least to detrimental damage of the equipment.